Absorption of Ultrasonic Waves by Various Gases

Abstract

Absorption coefficient for ultrasonic waves in C${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{H}}_{2}$ and He.---A beam of ultrasonic waves with a frequency of 612 kilocycles from an oscillating quartz crystal was passed through a mixture of gas and air, contained in a brass absorption tube with ends covered by thin celluloid films. The design of the tube greatly reduced the effect of resonance and allowed the use of two different lengths of absorbing gas, leaving the geometrical arrangement of openings and absorbing screens the same. The emergent beam was allowed to fall upon another quartz crystal which had a natural frequency very nearly equal to the frequency of the waves and which was connected to the grid of a triode. The intensity of the received signal was measured by a previously calibrated vacuum tube voltmeter. A logarithmic decrease in the transmitted energy with increase in the percentage of the gas in the mixture was found with C${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$O, ${\mathrm{H}}_{2}$, and He. The increase in the absorption coefficient (${\mathrm{cm}}^{\ensuremath{-}1}$) at 612 kilocycles when air was replaced by a mixture containing 1 percent by volume of the gas was found for C${\mathrm{O}}_{2}$ to be 0.029; for ${\mathrm{N}}_{2}$O, 0.034, for ${\mathrm{H}}_{2}$, 0.014, for He, 0.0025. For argon mixtures no absorption was observed.Reflection of ultrasonic wave by a thin celluloid film. The fractional part of the beam which was transmitted by the films at the ends of the absorption tube, increased with increase in percentage of C${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$O but decreased with increase in percentage of ${\mathrm{H}}_{2}$ and He, agreeing qualitatively with Rayleigh's theory.